A voltage controlled oscillator (VCO) is often based on an inductive/capacitive component, often referred to as an LC-tank, which is used to set the frequency of the VCO.
In VCOs based on LC-tanks, it is known to control the frequency digitally, for example using switching transistors, for example MOSFET switches, that are used to connect or disconnect one or more capacitor cells forming part of a capacitor array. In this way the effective capacitance value of the LC-tank can be controlled or varied in order to set the frequency of the VCO.
Differential capacitor switches are often used, for example where a MOSFET is AC coupled, and thus a DC bias is required. FIG. 1 shows an example of such a known circuit, whereby a resistor R is used to bias the MOSFET switch T0, the MOSFET switch T0 being coupled between first and second capacitors C0A and C0B. The first and second capacitors C0A and C0B together form a capacitor cell C0 of a larger capacitor array that comprises one or more similar capacitor cells C0 to CN (the other capacitor cells of the array not shown for clarity). One or more of such capacitor cells C0 of the capacitor array can be selectively coupled in parallel to an inductor L. In this way the capacitance value of the LC-tank can be controlled or selected, which in turn can be used, for example, to control the frequency of a voltage controlled oscillator.
The use of a bias resistor R for providing a DC bias for the MOSFET switch T0 has the following disadvantages. One the one hand the resistance value of the bias resistor R has to be large enough so that it does not load the LC-tank. However a large value of R increases chip area. On the other hand a small value of R has the effect of reducing the quality factor Q, whereby Q relates to the ratio of the reactance of a capacitor to the resistance of the capacitor at a given frequency. However, the phase noise is proportional to 1/Q2, thus a low Q is undesirable, since it has the effect of increasing the phase noise. Also, for a fixed amplitude the power consumption is proportional to 1/Q.
The bias resistor R should be much larger than the equivalent parallel resistance Rp of the LC-tank, and for a 5 bit binary sized capacitor switch (i.e. a capacitor array comprising 5 capacitor cells C0 to C5), the bias resistor R for the least significant bit should also be 16 times larger than the most significant bit, thus 5 resistors with the value of 16x, 8x, 4x, 2x, x, wherein each resistor in turn should be significantly larger than Rp. As a consequence, these biasing resistors tend to take up a very large chip area.